DS, if you can't find some good games, make em ;P http://www.dsgamemaker.com/
Although to do that you'd still need something that lets you run homebrew like R4 or something...
Edit: Ah! he might want to look at the GP2X actually http://gp2x.co.uk/ very awesome handheld linux console/pc and comes with all the dev ware to make stuff for it as well. Although, if you wait for their other product, Open Pandora http://openpandora.org/, it's going to be even better =D
Edit 2: Seems Open Pandora is selling right now, just limited due to it still being updated, but this is probably all you need to hear [quote:1of6cahb]How powerful is it?
It is by far the most powerful handheld in the world both in terms of raw CPU power and 3D graphics capability, it will be able to handle things such as Firefox3 or Quake3 with ease.